1. Field of the Invention
The present invention relates to a carbazole compound and a light-emitting element using the carbazole compound. The present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is probably suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight and has very fast response speed.
Further, since such a light-emitting element can be manufactured to have a film shape, surface light emission can be easily obtained. Therefore, a large-area element using the surface light emission can be formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is extremely effective for use as a surface light source applicable to lighting and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
There are many problems which depend on a substance in improving element characteristics of such a light-emitting element. In order to solve the problems, improvement in an element structure, development of a substance, and the like have been conducted. For example, Patent Document 1 discloses a light-emitting element in which a compound having an anthracene skeleton and a carbazole skeleton is used as a light-emitting material. However, it cannot be said that the light-emitting element has sufficiently high reliability.
Further, Patent Document 2 discloses a light-emitting element in which a compound which has an anthracene skeleton including a substituted or unsubstituted phenyl group and a carbazole skeleton and has an excellent carrier-transport property is used. The light-emitting element has low drive voltage and has high reliability.